1. Technical Field
This invention relates in general to integrated circuit packaging and, more particularly, to packaging micromechanical devices with reduced stiction and mechanical memory.
2. Description of the Related Art
Electronic devices such as integrated circuits are often packaged in hermetically sealed enclosures. These enclosures protect the device from contaminants, particles, and water vapor that would otherwise enter the package and mechanically damage or electrically disrupt the device. MEMS (microelectromechanical system) devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching which have been developed for the fabrication of integrated circuits.
Micromirror devices, sometimes referred to as DMDs (digital micromirror devices), are a type of MEMS device. Micromirror devices are primarily used in optical display systems. In display systems, the micromirror is a light modulator that uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, micromirrors typically operate in a digital bistable mode of operation and as such are the core of the first true digital full-color image projection systems. Other types of micromechanical devices include accelerometers, pressure and flow sensors, gears and motors. For some micromechanical devices, such as DMDs, it is important to maintain a very specific moisture level within the package, where the moisture level remains relatively steady over time.
Getters are compounds that capture contaminants, moisture vapor, and particles, are included inside the device enclosures to trap these species and preclude degradation of device performance, thereby increasing the operational lifetime of the device. Various getter compounds are available depending on the environment to which the getter will be subjected. The getter compound used in a DMD sinks moisture when the moisture level is too high and sources moisture when the moisture level is too low.
Many micromechanical devices include moving components that place unique demands on surface lubrication and passivation systems. For example, the deflectable element of a micromirror device rotates about a torsion beam hinge axis and is stopped by contact with a landing zone or spring structure. The contact point experiences metal-to-metal contact and some scrubbing action. This metal-to-metal contact can create static friction (stiction) between the components. Stiction is caused by the capillary action of water vapor present on the surface, van der Waals attraction, and intermetallic bonding of the metals. Stiction becomes worse as the contacting surfaces wear against each other since the contact area is increased.
Passivation coatings on micromirror devices reduce stiction and wear between the contacting surfaces. One passivation material that is especially useful for micromirror devices is a perfluoroalkanoic acid such as perfluorodecanoic acid (PFDA). PFDA, as taught by U.S. Pat. No. 5,331,454, issued Jul. 19, 1994 and entitled Low Voltage Reset for DMD, which is incorporated by reference herein, provides an oriented monolayer on the surfaces of the DMD. The oriented monolayer provides a chemically inert surface that reduces the stiction between adjacent metal parts.
While the PFDA reduces stiction problems, it would be beneficial to reduce stiction further. Therefore, there is a need for an improved method and system reducing stiction in a MEMS device.